The present invention relates to a method of manufacturing an inspection unit for a high frequency/high-speed device for ensuring reliable connection between the inspection unit and the device to be inspected, on occasion of inspecting its electrical performance, before a module of a high frequency/high-speed circuit such as an amplifier circuit, a mixer circuit, a filter circuit, a memory, a CPU, etc. or an IC to be incorporated in a mobile phone, for example, has been assembled to a circuit board. In this specification, the term “high frequency” refers to an analogue signal having a high frequency (1 GHz or more), while the term “high-speed” refers to a digital signal having very short pulse width and pulse interval, and both of which are hereinafter collectively referred to as RF (radio frequency).
On occasion of inspecting electrical performance of the RF device such as a semi-conductor wafer, an IC, or a module, insufficient contacts between the terminals may particularly cause fluctuation of impedance or other measurement factors, which may sometimes vary to change measured values. Under the circumstances, such inspection has been conducted by a special inspection unit, for example, as shown in FIG. 6 (disclosed in Japanese Patent Publication No. 2001-99889A). In such an inspection, an RF circuit, which is the device to be inspected, is constructed in a form of a module 50 including an amplifier circuit and a mixer circuit, and is housed in a metal casing for avoiding interference with the exterior. The module 50 includes input and output terminals 51, 54 for RF signals, a power supply electrode terminal 52, and a grounding terminal 53, which are provided on a back face of the metal casing. Then, the inspection is conducted by electrically connecting the terminals to respective terminals of a wiring board 66 on which certain wirings for the inspection are arranged.
In the example as shown in FIG. 6, there are employed contact probes each having a spring and a plunger contained in a metal pipe, one end of the plunger being adapted to be projected to the exterior by the spring and contracted when pushed. The respective electrode terminals are connected by contact probes 63 for RF signals, a contact probe 64 for power supply, and a contact probe 65 for grounding which are contained in a metal block 61 for preventing them from being affected by noises. Each of the contact probes 63 for RF signals is formed in a coaxial structure, using the contact probe as a core conductor and using an inner wall of a through hole in the metal block 61 as an outer conductor, especially for preventing intrusion of noises. In FIG. 6, denoted with numeral 67 is a coaxial cable, and 68 is a plate for pressing the metal pipes surrounding the contact probes. Such a structure around the contact probes is almost the same in a case where an IC socket for inspecting ICs, though such socket has a different outer shape.
Although FIG. 6 shows only two contact probes 63 for RF signals (for input and output), and one each contact probes 64, 65 each for power supply and for grounding, a large number of these contact probes are actually provided in the metal block 61. In the maximum case, the electrode terminals of about 600 pieces per 1 cm2 are provided in a matrix manner with a narrow pitch of about 0.4 mm.
In such the narrow-pitch device, an outer diameter of the contact probe for RF signals including a dielectric layer must be reduced in size. Meanwhile, it is also necessary to adjust the impedance of the coaxial structure formed by the contact probe and the inner wall of the through hole to a predetermined characteristic impedance (50Ω, for example) satisfying the following Equation (1).
                    Zo        =                              60                                          ɛ                r                                              ⁢                      log            e                    ⁢                      D            d                                              (        1        )            where, d is the outer diameter of the core conductor (the contact probe), D is the inner diameter of the outer conductor (the through hole), and εr is a dielectric constant of the dielectric substance between them.
In order to satisfy the Equation (1), it is possible to reduce the inner diameter D of the outer conductor by providing a tube made from dielectric substance with small dielectric constant between each contact probe and each through hole. However, even though a tube of polytetrafuluoroethylene having dielectric constant of 2.1, which is the dielectric substance having the smallest dielectric constant available at present, is employed, and the contact probe having the smallest diameter available (having the outer diameter of 0.15 mm) is employed, the inner diameter of the outer conductor (the inner diameter of the through hole formed in the metal block) requires about 0.5 mm to obtain 50Ω as the characteristic impedance of the coaxial structure This cannot attain the pitch of 0.4 mm.
For the purpose of solving the problem as described above, in a related-art structure, a hollow space is formed between the contact probe and the through hole to obtain the specific dielectric constant of approximately 1, thereby narrowing the pitch. As a retainer for retaining the contact probe at a center of the through hole, in the related-art structure, there are provided metal plates 72 on both faces of a metal block 71 and inserting insulating spacers 73 respectively into dented parts 72a which are formed in the metal plates 72, as shown in FIG. 5A, thereby to fix the contact probe 10. According to this structure, it is possible to make the diameter of the contact probe to be about 0.15 mm, and the inner diameter of the insertion hole 71a to be about 0.35 mm, which can be applied to the pitch of 0.4 mm.
However, this insulating spacer 73 is very small in size, having a thickness t of about 0.6 mm, an outer diameter A of about 0.33 mm, and an inner diameter B of a dented part of about 0.17 mm, as shown in FIG. 5B, and has poor workability. It requires a large number of working steps to accurately produce such a small product having a diameter less than 0.8 mm, either by machine work or by molding work. Moreover, it is difficult to insert the insulating spacer 73, which has been separately prepared, into the dented part 72a of the metal plate 72 and fix it therein. In addition, there is a problem of cost-up, because the production of the insulating spacer 73 is very difficult in view of mechanical strength of the insulating spacer 73. Further, in tendency of narrowing the pitch, it is impossible to prepare the insulating spacer 73 which has a smaller wall thickness than ever. Not only in a case where the contact probe for RF signals in which the coaxial structure is employed, but also in a case where the contact probe for power supply, it is necessary to employ the contact probe having as large diameter as possible for the purpose of decreasing a resistance loss, and the wall thickness of the insulating spacer 73 must be reduced to the limit, so as to comply with the tendency of narrowing the pitch.